Characteristic features of low-temperature structural phase transitions in La₂-_xSr_xCuO₄

Features of low-temperature structural phase transitions in La₂-_xSr_xCuO₄ with x=0.115 and 0.15 have been examined by transmission electron microscopy. There are 100-type forbidden spots due to the Pccn/LTT phase as well as fundamental spots due to the LTO phase in the electron-diffraction patterns of La₂-_xSr_xCuO₄ with x=0.115 and 0.15 at 12 K. As for 100-type forbidden dark field images at 12 K, a bright-line contrast due to the Pccn/LTT phase exists only along the twin boundary in the LTO phase for x=0.15. In the case of x=0.115, a bright-spot contrast due to the Pccn/LTT phase also appears partially inside the LTO domains, in addition to the bright-line contrast along the twin boundary. That is, the low-temperature transition in X=0.115 is characterized by the appearance of the Pccn/LTT phase with the spotty shape inside the LTO domain.